First things first, are what you are trying to underline in a body of text, or a actual flash button?
If they are in a body of text you can just change that using css and inserting the next as html or xml, if they are buttons, i would recommend making a graphic file that is the underline and placing it in each button file (adjusting the length accordingly) - time consuming, but prolly the fastest way to do this...
Cheers
email me the FLA if you want, ill take a look (michaeltighe@gmail.com)